An expandable polymer is any polymer capable of being expanded or stretched where it may become a porous membrane. One example of an expandable polymer is polytetrafluoroethylene (PTFE). PTFE is a fluorocarbon solid, as it is a high molecular weight compound consisting wholly of carbon and fluorine. It is an extremely versatile synthetic polymer due to its chemical inertness, wide temperature range, low friction coefficient and biocompatibility.
Expanded PTFE or ePTFE, discovered by W. L. Gore (U.S. Pat. No. 3,664,915), is PTFE that has been stretched to make it porous after being first extruded by a process called paste extrusion. Expanded PTFE has the inherent properties of PTFE, plus it is porous, and has higher strength than PTFE due to the fiber network formed during processing. ePTFE is usually available as a tape, film, or membrane type material.
ePTFE, due to its inertness, wide chemical compatibility and temperature range properties, may be ideal for many medical and industrial applications. Extremely thin and high strength variants of ePTFE are useful for some medical and industrial applications. In the case of endovascular devices, thinner wall, lower friction, higher strength membrane tubing may be desired due to the small diameter arteries that the devices must pass. Acoustic devices, including cell phone speakers and hearing aid device diaphragms, may require high strength flexible membranes and a venting function in order to relieve pressure buildup due to the thermal expansion of air in the device or barometric pressure changes. An industrial application such as a fuel cell MEA requires a high strength ePTFE membrane that has minimal creep and swelling. In the case of a gas or liquid filter that is laminated to a nonwoven or polymer film which is porous or perforated, it is useful to have an ePTFE membrane that has a slick low friction surface that also enables a high strength lamination bond to the nonwoven or polymer to which it is being laminated. The slick membrane surface minimizes filter cake buildup, while the high bond strength helps prevent failure of the laminate, which is reversed to release filter cake buildup. For certain medical device, filtration, chemical cell, or fuel cell applications it is useful to have an ePTFE membrane that bonds well to polymer films so as to provide a thermal melt sealable surface, or to provide a high integrity non porous gasketing surface on the perimeter of the membrane for improved sealing. In the case of solar cells or solar heating systems, UV water and air purification systems an ePTFE membrane with increased light transmittance at wider incidence angles may improve efficiency or performance. For lighting devices, a light diffuser that has a high transmittance, is breathable, and can sustain higher temperatures than other polymers may be desirable. Thus, there is a need to create an ePTFE with unique properties for both medical and industrial applications.
A conventional method of forming an article made of an expandable polymer, such as ePTFE, is to blend a powdered resin with a wettable liquid, such as a lubricant or extrusion aid, and compress the combination into a preformed mold, typically of a cylindrical shape. The wettable liquid may be mixed with the powdered resin to control the degree of material shear that occurs during subsequent extrusion and to prevent excessive shear, which can damage the material. Using a ram-type extruder, the mold may then be extruded through a die having a desired cross-section. The extruded material may be calendered or compressed after extruding to reduce the cross section. Next, the wettable liquid is removed from the extruded material by evaporating the wettable liquid through drying or any another extraction method. The dried extruded material may then be stretched in one or more directions at an elevated temperature below the crystalline melting point of the resin. Finally, the stretched material is typically sintered, or heated to a temperature ranging from just below to significantly above its melting point, depending on the time at the elevated temperature, in order to lock in the physical state of the material.
U.S. Pat. No. 6,890,463 is directed to a method involving rewetting of expandable polymers with a liquid to allow for subsequent enhanced expansion. The wetting liquid includes a drug and/or an agent, such that the resulting polymer contains and emits the drug upon positioning. This method is said to provide the material with unique properties. However, the method teaches that a stretching step be performed after the application of the wetting liquid. Thus, this method is limited to applying the wettable liquid before the expansion step.
However, the addition of a wetting liquid, with or without the inclusion of a drug or active agent, to mat down or densify the material, right after expansion and without any subsequent expansion, prior to high temperature sintering, may be advantageous in providing a material with unique properties. The instant invention is designed to address this issue.